Field of the Invention
The present invention relates to image processing of acquiring measurement values of color patches using an imaging apparatus.
Description of the Related Art
A display screen is measured using a spectral radiancemeter. It is possible to measure only one point on the screen by measurement using the spectral radiancemeter. If a number of color patches displayed on the screen are measured, it is necessary to perform the measurement by the number of colors. This requires much labor and time, thereby imposing a heavy load on the user who performs measurement. In recent years, a simple method of measuring a display screen using a digital camera capable of two-dimensionally acquiring measurement values may be adopted.
On the other hand, as for displays, displays having a high contrast ratio, such as a liquid crystal display (LCD) having a light emitting diode (LED) back light and an organic electroluminescence display (OLED), have made their debut. When a digital camera is used to measure such display screen, the dynamic range of the display may exceed that of the digital camera to cause highlight-detail losses and shadow-detail losses, resulting in difficulty in measurement within a range exceeding the dynamic range of the digital camera.
Japanese Patent Laid-Open No. 2006-118991 discloses a method of measuring a display screen using a digital camera by avoiding highlight-detail losses and shadow-detail losses. This method changes a capturing condition (shutter speed or f-number) according to the brightness of the screen so that pixel values after image capturing fall within a desired range (from 50 to 200 in 8 bits). The pixel values are then converted into luminance values by an approximation based on the capturing condition and constants depending on the digital camera.
However, under different capturing conditions, processes for optical black pixels in the digital camera and parameters for noise removal and the like are different. Using only a simple gain calculation or one approximation cannot obtain accurate luminance values from the pixel values of the digital camera.
Furthermore, since the pixel value and the luminance value have no complete linear relationship, the relationship between the pixel value and the luminance value may be discontinuous between data obtained under different capturing conditions to cause reversal of tonality, thereby disabling obtaining of an accurate measurement value (luminance value).